Automatic swimming pool cover with a dual hydraulic drive system

ABSTRACT

A hydraulic drive system is described for automatic swimming pool cover systems in which a first hydraulic drive provides torque for both resisting cover drum rotation during cover extension across the pool and rotating the cover drum for cover retraction, while a separate second hydraulic drive provides torque for both rotating the cable reels for cover extension and resisting cable reel rotation during cover retraction.

Related Applications

This application is a continuation-in-part of application Ser. No.07/494,564 filed Mar. 16, 1990, now U.S. Pat. No. 5,067,184 in theUnited States of America by the applicant, Harry J. Last, entitled "ACOVER DRUM HAVING TAPERED ENDS FOR AN AUTOMATIC SWIMMING POOL COVER"which is a continuation-in-part of Ser. No. 07/258,000, filed Oct. 17,1988, now U.S. Pat. No. 4,939,798 issued Jul. 10, 1990 to applicant,Harry J. Last, entitled: "LEADING EDGE AND TRACK SLIDER SYSTEM FOR ANAUTOMATIC SWIMMING POOL COVER".

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to automatic swimming pool cover systems, and inparticular, to the drive systems for rotating the cable reels and coverdrum for extending and retracting pool covers back an forth across aswimming pool.

2. Description of the Prior Art

Automatic swimming pool cover systems typically include a flexible vinylfabric sized so that most of it floats on the surface of the pool water.The pool water acts as a low friction surface significantly reducing theamount of force required to move the cover across the pool. The frontedge of the cover is secured to a rigid boom spanning the width of thepool for holding the front edge of the cover above the water as it isdrawn back and forth across the pool.

To draw the cover across the pool, a cable, typically a Dacron line, isincorporated into and forms a beaded tape which is sewn or attached tothe side edges of the pool cover. The beaded tape in turn is capturedand slides within a "C" channel of an extruded aluminum track. The trackis secured either to the pool deck or the the underside of anoverhanging coping along the sides of the swimming pool. The cablesextending from the beaded tape sections of the cover are trained aroundpulleys at the distal ends of the tracks and return in a parallel "C"channel to the drive mechanism where they wind around cable take-upreels.

To uncover the pool, the drive mechanism rotates a cover drum mounted atone end of the pool winding the pool cover around its periphery andunwinding the cables from around the take-up reels. To cover the poolthe drive mechanism rotatably drives the cable take-up reels winding upthe cables to pull the cover across the pool unwinding the cover fromaround the cover drum.

The rate at which the pool cover unwinds from and winds onto the coverdrum varies depending on the diameter of the roll of the cover stillwound around the drum, i.e., the rate is greatest when most of the coveris wound around the drum (largest diameter) and least when the cover ispractically unwound from the drum (least diameter). The same phenomenonoccurs as the cables wind onto and unwind from the cable reels. Itshould be appreciated that the cables wind onto the cable reels at thehighest rate when the cover unwinds from the cover drum at its lowestrate and visa-versa.

In systems where the cable take-up reels and the cover drum rotatetogether on the same axle, but oppositely wind/unwind the cables andcover respectively, a spring is utilized as a tensioning take-upmechanism to compensate for the different and varying rates at which thecables and pool cover wind and unwind from the respective reels and drumduring the opening and closing cycles. The spring mechanism lengthensand shortens the cable path as the cover is drawn back and forth acrossthe pool taking up and yielding slack in the respective cables asnecessary to compensate for the differences in the winding and unwindingrates of the reels and drum. [See U.S. Pat. Nos. 3,747,132 and3,982,286, Foster.]

In spring tensioning take-up systems of the type described by Foster,and later floating spring tensioning take-up systems of the typepioneered by Last, the applicant herein, the tesioning of the cables bythe spring(s) assures that the cover, and especially its beaded edgescurling around the ends of the drum, wind tightly and uniformly withoutsubstantial bias around the cover drum as the cover is retracted fromacross the pool. However, there is an upper limit beyond which thetensioning/compensating spring of such single axle systems can notcompensate for the differential in winding rates of the cover andcables. [See U.S. Pat. No. 3,982,286, Foster, Col. 5, 1. 36-Col. 6, 1.4. See also related U.S. Pat. No. 4,939,798.]

In other systems, a clutching mechanism is typically utilized todecouple the rotation of the cable reels from that of the cover drum asit is rotatably driven to wind the cover onto the drum uncovering thepool, and to decouple the rotation of the cover drum from that of thecable reels as they are rotatably driven to draw the cover across thepool. Typically, in such systems, the cable reels are allowed to freewheel when the cover drum is rotatably driven and conversely, the coverdrum to free wheel when the cable reels are rotatably driven. [See U.S.Pat. Nos. 3,019,450 and 3,050,743, Lamb.]

In clutch decoupled systems of the type pioneered by Lamb, in order toprevent biasing of the cover as it winds around the cover drum duringretraction and to assure that the cover winds compactly and uniformlyaround the drum, adjustable braking mechanisms are utilized to slow orresist rotation of the respective free wheeling take-up reels to providethe necessary tension in the cables for assuring that cover edges curlaround the ends of the cover drum. Such braking mechanisms typically areadjustable for each take-up reel.

In early automatic pool cover systems the rigid boom spanning the widthof the pool holding the front edge of the cover above the water wastypically supported by a pair of wheeled dollies rolling on the sideedges of the pool. The cables moving within the "C" channels of thetrack along either side of the pool were either directly secured in somefashion to the rigid boom, [Foster, supra], or were indirectly securedto the ends of the boom via fabric interfaces referred to as gores. [SeeU.S. Pat. No. 4,001,900, Lamb].

Slider mechanisms have supplanted the use of wheeled dollies forsupporting the rigid boom carrying the front edge of the cover.Typically, such slider mechanisms are coupled to the respective ends ofthe boom and have an edge adapted for capture and sliding within thesame or different "C" channels of the extruded track in which the beadedside edge of the cover is captured and slides. [See U.S. Pat. No.4,686,717, MacDonald et al and U.K. Patent No. 2,072,006, Lee.]

As pointed out and extensively discussed in Applicant's related U.S.Pat. No. 4,939,798, in systems where slider mechanisms support the rigidboom, it is very important to maintain the boom oriented squarelybetween the track channels, otherwise the sliders carrying the boom willjam in the track channels stopping extension or retraction of the cover.Even with wheel supported booms, any canting during extension orretraction will tend to pull the beaded cover edge free of the confiningtrack channels particularly at its front corners.

SUMMARY OF THE INVENTION

The invented hydraulic drive for swimming pool covers systems includes afirst reversible hydraulic motor mechanically coupled for rotating cablereels around which the cables, extending from beaded side edges of apool cover, wind and unwind, a second reversible hydraulic motormechanically coupled for rotating a cover drum around which the poolcover winds and unwinds, a source of hydraulic power, and a controlmeans hydraulically coupling the respective motors to the source ofhydraulic power for: (i) providing a driving torque, via the firstmotor, for rotating the cable reels winding up the cables whilesimultaneously providing a resistive torque, via the second motor,resisting rotation of the cover drum as the cover unwinds and is drawnacross covering the pool; and (ii) providing a driving torque, via thesecond motor to wind the cover around the cover drum retracting it fromacross uncovering the pool, while simultaneously providing a resistingtorque, via the first motor, resisting rotation of the cable reels totension the cables and cover as the cover retracts and winds around thecover drum.

In particular, the respective reversible hydraulic motors each functionas both a motor and a pump and are mechanically coupled by theinterconnecting cables and cover winding and unwinding from around therespective cable reels and cover drum such that when one motor ishydraulically driven to provide torque, the other motor hydraulicallyresponds as a pump. The hydraulic exhaust from the driving motorprovides hydraulic input for the pumping motor. The control meansincludes at least a two position hydraulic valve for reversing flow ofhydraulic liquid from the source of hydraulic power through therespective motors/pumps.

A particular novel and advantageous feature of the invented hydraulicdrive system is that limit switches for interrupting coverextension/retraction can be eliminated by appropriate adjustment ofinput and output pressure relief valves which limit the driving torqueavailable for rotating the cable reels and the cover drum during coverextension and retraction, respectively, such that an increase in torqueload above a threshold value stops extension/retraction.

Another advantage of the invented drive system is that a tension load onthe cover and cables can be established and then maintained relativelyconstant as the cover extends and retracts, as well as when the cover isat rest, in the fully extended and retracted positions, in contrast, tospring tensioned, single axle systems in which tension increases anddecreases as the cover extends and retracts back and forth across thepool, and in contrast to clutch de-coupled systems in which can notmaintained tension on the fully extended and/or retracted cover.

Accordingly, a particular object of the invented drive system is controlover the tension in the cables and cover during cover extension andretraction achieved by increasing and decreasing pressure in the outputline from the motor functioning as a pump using an adjustable pressurerelief valve.

Still another advantage of the invented drive system is that thedifferential in winding/unwinding rates of the cover and cables do notimpose an upper limit on the length of pool that can be covered anduncovered by the automatic cover system.

Other advantageous features of the invented drive system relates toelimination of electrical power hazards in the the pool environment inthat electrically driven hydraulic power sources can be locatedremotely.

Another aspect of the invented drive system is that the cover drumdisposed at one end of the pool can be located in a trench flooded withpool water to provide buoyant support to the cover drum and cover drumroll for counterbalancing bending moments on the cover drum due to theweight of the cover wound around it.

Still other features, aspects, advantages and objects presented andaccomplished by the invented hydraulic drive system for automaticswimming pool covers will become apparent and/or be more fullyunderstood with reference to the following description and detaileddrawings of preferred and exemplary embodiments.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan schematic view of an automatic swimming pool coversystem incorporating and illustrating the essential components of theinvented dual hydraulic motor drive system.

FIG. 2 is a side elevation schematic the cover drum with the pool coverwound around it disposed in a flooded trench illustrating thecounterbalancing of the buoyancy forces and the bending moments on thedrum.

DESCRIPTION OF PREFERRED AND EXEMPLARY EMBODIMENTS

Referring to FIG. 1, a top plan view of an automatic pool cover systemis shown which includes a leading edge and slider system of the typedescribed in Applicant's U.S. Pat. No. 4,939,798, entitled "Leading Edgeand Track Slider System for an Automatic Swimming Pool Cover" and withconically tapered hubs at either end of the cover drum as described inApplicant's co-pending U.S. application Ser. No. 07/494,564, entitled,"A COVER DRUM HAVING TAPERED ENDS FOR AN AUTOMATIC SWIMMING POOL COVER."

As shown in FIG. 1, a flexible vinyl fabric pool cover 11, is attachedfor winding around a cylindrical cover drum 12 with conically taperingend sections or hubs 13 supported for rotation at the end of a swimmingpool (not shown). The front edge 14 of the cover 11 is supported by arigid leading edge 15 spanning the width of the pool above the waterbetween conventional parallel "C" channel swimming pool tracks 19secured along the sides of the swimming pool. Sliders 20 coupled at eachend of the rigid leading edge are fastened to cables 21. The sliders 20are captured and slide within the "C" channels of the respective tracks19. [See U.S. Pat. No. 4,939,798.]

The cables 21, typically Dacron lines, are incorporated into and form abeaded tape 22 sewn to the side edges of the cover 11. The cables 21extend from the front corners of the cover 11, are trained aroundpulleys 23 at the distal ends of the tracks 19, and return via returninternal "C" channels within the track 19 to ultimately connect with andwind onto a pair of cable take-up reels 16. Return pulleys 25 providethe necessary changes of direction between the return channels of thetracks 19 and the cable reels 16. The beaded tapes 22 sewn to the sideedges of the cover 11 are captured and slide within the conventional "C"channels within the respective tracks 19.

A floating pair of coupled pulleys 61 are incorporated into therespective cable paths 62 between the take-up reels 16 and tracks 21 tocompensate for differences in the rates at which the cables 21 windaround the respective cable take-up reels 16 during cover extension. Inparticular, the diameter of the cables winding around the respectivetake-up reels 16 frequently differ, depending on the distribution of thecable coil layers around the reel. More cable 21 is wound around thereel 16 of the larger diameter than the smaller in a single rotation, afact which could cause the boom 15 to skew and jam between the "C"channels of the track 21.

The couple pulleys 61 translate toward the larger diameter take-up reel16 lengthening the cable path for the cable 21 being wound around thesmaller diameter take-up reel 16 thereby counter balancing orcompensating for the difference in cable lengths being wound around therespective take-up reels in any single rotation. The coupled pulleysalso equalize the tension in the respective cable paths.

A tension spring coupling between the pulleys will further improveperformance the coupled pair of pulleys 61 in the manner previouslydescribed in context of the automatic pool cover systems utilizingfloating spring tensioning take-up mechanisms. (See U.S. Pat. No.4,939,798.) To explain, the maximum tension load on the respective cablepaths occurs upon initiation of the extension cycle. The drivemechanism, via the cables, must overcome the inertial resistance of thecover drum with a fully wound up cover in addition to the frictionalresistance of the sliders and beaded tape edges in the "C" channels ofthe track 19. Accordingly, upon initiating cover extension, therespective cable paths experience shock loading. Such shock loading cancause mechanical and fatigue failures. Incorporating a tensioning springbetween the coupled pair of pulleys 61 provides necessary resiliency inthe cable paths to prevent such shock loading, and at the same time,provides a mechanism for increasing tension load on the cable paths toovercome the initial inertial resistance of the fully loaded cover drum.

The cover drum 12 is supported for rotation between the respectivetracks 19, within a trench located at one end of the pool (FIG. 2) by atranslating bearing block 28 and a bearing block (not shown) receivingand supporting axles 24 and 27 coaxially extending from the conical hubs13 at either end of the cover drum 12. Additional bearing blocks can bejournaled around to the respective supporting axles 24 and 27 to provideadditional capacity (and/or rigidity) for mechanical supporting thecover drum 12. However, care should be exercised in locating suchadditional bearing blocks to insure a desired range of axial translationof the cover drum drive train.

The cover drum drive train including the conical hubs 13 and axles 24and 27 can be translated along the longitudinal rotational axis of thecover drum 12 utilizing the translation bearing block 28. In particular,the bearing block 28 includes a helically threaded shaft 29 threadedthrough the rear wall of a rigid hexahedral frame 31. The end of theshaft 29 is mechanically coupled by a conventional non-rotating collar32 to a translating frame 33 supporting a bearing receiving the axle 27.An adjustment knob or crank 34 is mechanically fastened to the oppositeend of the shaft 29 extending out of the hexahedral frame 31.

To adjust the position of the cover drum drive train between theparallel tracks 19, the tension on the cover 11, and its associatedbeaded side edges 22 and cables 21 is relieved, typically, by releasinga ratcheting mechanism (not shown) allowing the cable reels 16 to freewheel on the reel axle 26. The bolts 36 securing the bearing frame 33within the hexahederal frame 31 of the translating bearing block 28 areloosened sufficiently to allow translation of the cover drum drivetrain. Shaft 29 is rotated with crank 34 by hand to translate the coverdrum drive train to a new position. The bolts 36 are then re-tightenedand the cover and cables re-tensioned by re-engaging the ratchetmechanism and turning either the cover drum 12 or cable reels 16.Diametric holes 37 located near the apex of the conical hubs 13 adaptedto receive a longitudinal bar or crank (not shown) for manually turningthe cover drum 12 can be utilized for re-tensioning the system.

As shown in FIG. 1, a first reversible hydraulic motor 17 ismechanically keyed to for rotating the end of axle 24 extending from thecover drum 12. A second reversible hydraulic motor 18 is mechanicallykeyed to for rotating axle 26 and the cable reels 16. Hydraulic lines 38and 39 connect between the respective motors 17 and 18 and a threeposition solenoid valve 41. Hydraulic line 42 connects between themotors 17 and 18. High pressure hydraulic input line 43 connects betweena pump 46 and the solenoid valve 41. Exhaust hydraulic line 44 connectsbetween the solenoid valve 41 and the hydralic fluid reservoir 47 via anadjustable pressure relief valve 48 and an anti-cavitation check valve50. Bypass hydraulic line 49 connects between the high pressure inputline 43 and the reservoir 47 via a second adjustable pressure reliefvalve 51. An anti-cavitation hydraulic line 52 connects between thehydraulic line 42 coupling the motors 17 and 18 and the exhaust line 44between pressure relief valve 48 and the anti-cavitation check valve 50.Overflow hydraulic line 53 connects between the anti-cavitation line 52and the reservoir 47. Low pressure check valves 54 and 56 are placed inthe anti-cavitation and overflow hydraulic lines 52 and 53 to assurehydraulic liquid flow through the anti-cavitation line 52 to the motors17 and 18 via line 42. The hydraulic pump is driven by an electricalmotor 57. A check valve 58 in high pressure line 43 prevents thehydraulic liquid from draining into the tank 47 when the pump 46 is off.A filter 59 is located between the pump 46 and reservoir tank 47.

The three position solenoid valve 41 is "normally closed" at position 1(as shown in FIG. 1) meaning that valve 41 does not allow hydraulicliquid flow to or from either of the motors 17 and 18 when notenergized, and hydraulic liquid circulates via the bypass line 49between the pump 46 and tank 47. When energized for extending the cover11, the solenoid valve 41 (at position 2) directs the high pressurehydraulic liquid from input line 43 to the hydraulic motor 18 coupled tothe cable reels 16 and exhaust liquid from motor 17 coupled to the coverdrum 12 to exhaust line 44. When energized for retracting the cover 11,the solenoid valve 41 (at position 3) directs the high pressurehydraulic liquid from input line 43 to the hydraulic motor 17 coupled tothe cover drum 12 and exhaust liquid from motor 18 coupled to the cablereels 16 to exhaust line 44.

The cover 11 winding and unwinding from around the cover drum 12 and thecables winding and unwinding from around the cable reels 16 provide amechanical connection between the respective motors 17 and 18. Inparticular, when motor 17 is driven by the hydraulic liquid for rotatingthe cover drum, motor 18 coupled to the cable reels is rotatably drivenas a pump. Similarly, when motor 18 is driven by the hydraulic liquidfor rotating the cable reels 16, motor 17 coupled to the cover drum 12is rotatably driven as a pump.

The torque provided to the driving motor must be sufficient to overcomeboth the inherent friction of the mechanical components of the poolcover system as well as the tension load imposed on the system by thepumping motor. It should be appreciated that the torque resistance whichmust be overcome by the driving motor increases with the "windingradius" of cover drum 12 or cable reels 16, and that tension loadimposed on the system by the pumping motor also increases as the the"unwinding radius" of the cover 11 around the cover drum 1 or cables 21around cables reels 16 decreases. The terms "winding radius" and"unwinding radius" refer respectively to the increases and decreases inradius due to layers of cover 11 being wound and unwound from around thedrum 12, in the case of the cover drum, and to the layers of cable coilsbeing wound and unwound from around the reels 16, in the case of thecover reels.

Sources of friction inherent in a pool cover system include bothconstant sources, and variable sources. The constant sources of frictionare those which do not vary as the cover extends and/or retracts, e.g.the friction of pulley system directing the cables 21 and the bearingssupporting turning axles, the cables moving in the return channels ofthe tracks 19 and the friction of the sliders supporting the leadingedge 15 sliding within the "C" channels of the tracks 19. Variablefriction sources are those that vary with the degree ofextension/retraction of the cover, e.g., the friction due to the beadedtape edges 22 of the cover 11 sliding within the "C" channels of thetracks 19 will increase as the cover 11 extends across the pool and willdecrease as the cover retracts.

From the above analysis, it should be appreciated that the tension loadimposed on the system by the pumping motor opposing the driving torqueof the driving motor approaches a maximum as the leading edge 15 of thecover 11 reaches the end points of its travel at either end of the pool.The resistance torque due to friction is at a maximum when the cover isfully extended.

Adjustable pressure relief valve 48 on the exhaust line 44 establishesthe exhaust pressure against which the pumping motor pumps, hence thetension load or resistance imposed on the system by the pumping motor.Adjustable pressure relief valve 51 in the bypass line 49 establishesthe pressure of the hydraulic liquid from the pump 46 and, hence, thetorque available to the driving motor for rotating either the cover drum12 or the cable reels 16. By appropriate adjustment of the respectivepressure relief valves 48 and 49, it is possible to counterbalance thedriving torque of the driving motor with the tension load imposed by thepumping motor for slowing and even effectively stoppingextension/retraction of the cover 11 as its leading edge 15 approachesits end points of travel.

In practice, however, to assure complete extension and retraction of thecover 11, the differential pressure of the driving and exhaust hydraulicliquids should always be adjusted such that the driving torque windingthe cables or cover will just exceed the maximum resistive torque. Stops61 located at the respective ends of the pool stop movement of theleading edge 15 are utilized to increase the tension load sufficientlyfor counter-balancing the torque of the driving motor. Such stops needonly be able to mechanically withstand the differential load of thedriving motor and the opposing tension load imposed by the pumpingmotor. (Such stops are inherent in under track pool cover systemscomprising the copings at the respective ends of the pool whichmechanically stop movement of the rigid leading edge 15.)

The differences between operational loads experienced during extensionand those experienced during retraction caused by, for example, theincrease/decrease of friction as a function of cover extension arecompensated through appropriate adjustment of the initialwinding/unwinding radius of the cables 21 (and/or cover 11). Forexample, the operational effect of increasing/decreasing friction duringcover extension/retraction can be offset by decreasing the initialwinding radius of the cable reels 16. In particular, since both thedriving and resistive (pumping) torques of the motor 18 coupled to thecable reels remain constant, the increase in friction load as the coverextends is offset by increasing the available winding force obtained bythe decrease in the cable winding radius. Similarly, the decreasingfriction load on cover retraction is offset by an increase in tensionload which again is obtained by decreasing the winding radius. And, itshould be appreciated that the winding radius of the cover increases asthe cover retracts causing the torque resisting windup to increaseoffsetting the decreasing component of resistance torque due to thedecreasing friction of the beaded tape edges 22 sliding in the "C"channels of the tracks 19.

Also, as discussed in co-pending U.S. application Ser. No. 07/494,564entitled "Cover Drum Having Tapered Ends For An Automatic Swimming PoolCover," the winding/unwinding radius of the cover roll 65 around thecylindrical cover drum 12 can be manipulated or adjusted as the coverwinds and unwinds by thickening sections of the cover with angularlyoriented strips of a suitable material such as foam (not shown). Theincrease in cover thickness due to such strips increases the radius ofthe cover roll 65 (FIG. 2) as a function of the position of the leadingedge 15 of the cover 12 relative to the cover drum 12. Such increases inthe winding/unwinding radius effectively increases the torque resistingwindup during retraction as the foam sections wind around the coverdrum. And, on cover extension, since the unwinding radius is greaterwhen such foam strips are still wound around the drum 12, less force(tension) is required in the cables and cover to cause the drum 12 andcover roll to unwind, i.e., less force (tension) is required forovercoming the torque resistance of the drum motor 17 acting as a pump.It is preferable to orient such thickening strips angularly with respectto the direction of travel of the cover such that the strips helicallywind and unwind from around the drum to mitigate strain caused bydifferential stretching in the cover fabric in the affected transverseregion of the cover over time.

Referring now to FIG. 2, because of the absence of electrical componentsin the pool environment, with the described hydraulic drive system, itis possible to flood the trench 62 in which the cover drum 12 is locatedat one end of the swimming pool 60 with pool water 63. A port 64communicates from the bottom of the cover roll trench 62 into theswimming pool 60 to allow pool water 63 to flow in and out of the trench62.

The buoyancy of the pool water 63 offsets the effect of gravityproviding lateral support to the cover drum 12 as the cover 11 windsaround it. In particular, for wide pools, the weight of the pool cover11 winding around the cover drum 12 can cause the drum 12 to sag or bendin the center creating twisting or torque moments on the respectivebearings supporting the drum 12 for rotation. Such twisting or torquemoments, being perpendicular to the rotational axis of the cover drumdrive train increase bearing friction and wear. In addition, excessivesag of the cover drum 12 inherently increases torque resisting windup inthat the effective windup radius of the cover around the drum increaseswith increasing sag. Finally, utilizing the buoyancy afforded by thepool water to provide lateral support to the cover drum and coverwinding around drum lessens mechanical rigidity and strengthrequirements for the components of the cover drum drive train with aresultant savings in materials costs and mass.

The invented dual hydraulic motor system for automatic swimming poolcovers has been described in context of both representative andpreferred embodiments. There are many modifications and variations canbe made to the invented drive system which, while not exactly describedherein, fall within the spirit and the scope of invention as describedand set forth in the in the appended claims.

I claim:
 1. A hydraulic drive system for extending and retractingswimming pool covers comprising, in combination,a first reversiblehydraulic motor mechanically coupled for rotating at least one cablereel around which a pair of cables, extending from side edges of thepool cover, wind and unwind, a second reversible hydraulic motormechanically coupled for rotating a cover drum around which the poolcover winds and unwinds, means coupling between the respective cablesfor equalizing tension in the respective cables and for compensating forany differential in rates at which the respective cables wind and unwindfrom around the cable reel, a source of hydraulic power, a control meanshydraulically coupling the respective motors and the source of hydraulicpower for:(i) providing a driving torque, via the first motor, forrotating the cable reel to wind the cables around the reel whilesimultaneously providing a resistive torque, via the second motor, forresisting unwinding rotation of the cover drum as the cover unwinds andis drawn across covering the pool; and (ii) providing a driving torque,via the second motor for rotating the cover drum to wind the coveraround the cover drum, while simultaneously providing a resistivetorque, via the first motor, for resisting unwinding rotation of thecable reels to tension the cables and cover as the cover retractsuncovering the pool.
 2. The hydraulic drive system for extending andretracting swimming pool covers of claim 1 wherein the first and secondreversible hydraulic motors are each driven as a pump to provide therespective resistive torques to the unwinding rotation of the cable reeland cover drum respectively, the cables and cover mechanically couplingbetween the motor providing the driving torque (the driving motor) andthe motor driven as a pump (the driven motor) providing the resistivetorque.
 3. The hydraulic drive system for extending and retractingswimming pool covers of claim 2 wherein the control means hydraulicallycoupling the respective motors and the source of hydraulic powerincludes:a) a reservoir of hydraulic luiquid hydraulically coupled tothe source of hydraulic power; b) a common hydraulic line couplingbetween the respective reversible hydraulic motors such that outputliquid from the motor providing driving torque is available as inputliquid to the motor functioning as the pump providing the resistingtorque; c) a first input/output hydraulic line connected to the firstreversible hydraulic motor; d) a second input/output hydraulic lineconnected to the second reversible hydraulic motor; e) a supplyhydraulic line connected to the source of hydralic power; f) an exhausthydraulic line conneted to the reservoir of hydraulic liquid; g) avalving means for hydraulically coupling the first input/output line tothe supply line while simultaneously hydraulically coupling the secondinput/output line to the exhaust line, at a first position, and forhydraulically coupling the second input/output line to the supply linewhile simultaneously hydraulically coupling the first input/output lineto the exhaust line, at a second position; h) a bypass linehydraulically coupling between the supply line and the reservoir; i) ahigh pressure adjustable pressure relief means in the bypass line forcontrolling hydraulic pressure in the supply line; j) a low pressureadjustable pressure relief means in the exhaust line for controllingpressure in the exhaust line; k) anti-cavitation means hydraulicallycoupling between the common hydraulic line and reservoir for assuringhydraulic liquid in the common hydraulic line.
 4. The hydraulic drivesystem for extending and retracting swimming pool covers of claim 3wherein the anti-cavitation means includes:l) a first check valve meanshydraulically coupled into the exhaust line between the low pressureadjustable pressure relief means and the reservoir for preventing fluidflow into the reservoir below pressure P₁ ; m) a second check valuemeans in a hydraulic line hydralically coupling between the common lineand the exhaust line upstream from the first check valve means forpreventing fluid flow to the common line below pressure P₂, where P₁ isgreater than P₂ ; and n) a third check valve means in a hydraulic linehydraulically coupling the common line and the reservoir preventingfluid flow from the common line to the reservoir below pressure P₂,whereby a supply of hydraulic liquid supply in the common line for thedriven motor is assured.
 5. The hydraulic drive system for extending andretracting swimming pool covers of claim 1 wherein the cover drum isimmersed in pool water at one end of a pool for providing lateralbuoyant support to the cover drum and cover winding and unwinding fromaround the cover drum.
 6. The hydraulic drive system for extending andretracting swimming pool covers of claim of claim 5 wherein the coverdrum and second reversible hydraulic motor coupled for rotating the drumare located within a trench at one end of the swimming pool and whereinat least one port communicates between the trench and the swimming poolfor allowing pool water to circulate into and flood the trench.